1. Field of the Invention
The present invention relates to a fixing device mounted on image-forming machines of the type of electrostatic photography, such as a copier, a printer, a facsimile and the like machine. More specifically, the invention relates to a fixing device comprising a fixing roller and two support rollers arranged maintaining a distance in a tangential direction of the fixing roller and wrapped with an endless belt, the support rollers being press-contacted to the fixing roller via the belt.
2. Description of the Related Art
A fixing device mounted on an image-forming machine requires a high temperature and a pressure applied for a predetermined period of time for fixing the unfixed toner onto a paper. In particular, a color image-forming machine, that is becoming ever popular in recent years, requires an increased amount of heat and an elevated pressure as compared to the monochromatic machines to print toners of a multiplicity of colors in an overlapped manner. To meet such a demand, it is advantageous to use a fixing device of the above-mentioned belt type having a large nipping width. A representative example of the above fixing device may be the one comprising a fixing roller (heat roller)and two support rollers wrapped with an endless belt and are press-contacted to the fixing roller via the belt (JP-A-2004-212844).
In the above fixing device, the one support roller is a pressing roller and the other support roller is a tension roller. The pressing roller is supported by a pressing roller support member capable of rotating about a shaft. The pressing roller support member is imparted with a rotational moment in one direction from a tension coil spring so as to rotate about the shaft, and the pressing roller is press-contacted to the fixing roller via the belt. The tension roller is supported by a tension roller support member that is supported by the pressing roller support member via a shaft so as to rotate. The tension roller support member is imparted with a rotational moment in one direction from a compression coil spring so as to rotate about the shaft, and the tension roller is press-contacted to the fixing roller via the belt.
In the above fixing device, a tension is imparted to the belt by a pressing mechanism which includes the tension coil spring and by a pressing mechanism which includes the compression coil spring. Further, the pressing roller and the tension roller are press-contacted to the fixing roller via the belt. Therefore, the entire constitution becomes complex. Besides, it is not easy to set a distribution (ratio) of the tension of the belt and the force of bringing the pressing roller and the tension roller into contact with the fixing roller via the belt.